Feynman diagrams illustrate the interactions between elementary particles. Originaly used to solve problems involving electrons and photons, Feynman diagrams are now used to describe all kinds of particle interactions.
Each diagram has a time axis and a space axis. If the arrow on the particle's line points opposite the direction of forward time, it indicates an antiparticle. In the diagrams, a solid external line represents a real particle. A dashed, wavy or spiraled internal line indicates a virtual particle or mediator particle. A wavy line indicates a virtual photon, mediator of the electromagnetic force. These interactions involve only leptons. A spiraled line is a virtual gluon, mediator of the strong force. These interactions involve only quarks. (See: Quantum chromodynamics.) A dashed line indicates a W+ or a W- particle, mediators of the weak force. These interactions can involve any type of particle